Significant NPCs:

Aboleth, possibly illusory

2 Leech Swarms

Huge Leech

Derro Skum Wizard

Quintus, Wizard/Alchemist

We started the session in mid-battle, though all that was left was the huge aquatic wizard monster (later identified as probably an Aboleth) and a leech swarm. (Some other leech swarms, several skum zombies, and a possibly illusory Roper had already been defeated.) The sea wizard was swimming (at insanely fast speed) toward an exit in the southeastern corner of the big pool room. The leeches were on the slippery sloping passage with the PCs, trying to suck some blood from anything warm-blooded.

Garreth got swarmed by the leeches, for significant but not terrifying damage, but he easily made his 2 HT rolls to avoid whatever follow-up effects the leeches caused. (His Resist Disease helped on one of the two rolls; his Resist Poison helped on neither. So one of the rolls was something else.) Elias, with Heroic Grace still active boosting his DX to 19 and Great Haste doubling his attacks per turn, closed in on the leech swarm and flailed them multiple times, finishing them off with a messy splat. Seépravir finally remembered to dispel the Levitate spell on Zaber, that she had cast for emergency evacuation purposes but which was now hanging him uselessly in the air, so he could move freely. Zaber easily made his Acrobatics roll, landed on his feet, and avoided sliding down the slippery passage. Zaber considered jumping in the pool, but then remembered that Seépravir had dropped the Warmth spell on him, and decided it wasn't a good day for hypothermia.

Polly leaned out over the edge of the ledge to get line of sight on the retreating sea wizard, and fired a couple more arrows at it. One was bounced aside, probably by a Deflect spell. Another sunk deeply into the creature's hindquarters, resulting in more green blood spurting out. Then the creature managed to retreat out of sight.

At that point the PCs decided not to swim across open water at a giant aquatic wizard monster, and instead to go back uphill for a skiff. Everyone managed to climb up the slippery slope, except Elias, who Garreth pulled up behind him with a rope. At that point Polly and Garreth both asked Elias for a Stop Bleeding spell, and then everyone took a little rest. Their usual wandering monster luck prevailed, and everyone managed to get all their FP back without getting ambushed.

At that point there was a discussion on whether to take the skiff down the waterslide passage (if it fit) to have a boat at the bottom, or use the Folding Boat down there instead, or leave that area alone for now and continue down the underground river. The eventual consensus was to drag the skiff downhill, saving the Folding Boat for emergencies rather than subjecting it to possible damage. Garreth and Elias managed to portage the boat down the narrow passage without falling over. When they got to the pool at the bottom, there were no enemies to be found.

At that point Zaber asked Seépravir to renew Warmth and Invisibility on him, so he could scout safely. He then did a couple of of slow laps around the pool, looking for secrets. He found a hidden underwater passage to the north, which the group decided not to explore yet. He also found a dry-ish passage to the southeast, featuring a green blood trail that went for a few yards then abruptly disappeared. Seépravir decided to cast Seek Magic, which found some magic nearby to the south, which was enough to convince everyone to go that way.

Zaber scouted ahead and saw that the room ended in a round room with a flat floor, but also (with his excellent and also magically-enhanced Perception) that the flat floor was an illusion and there was actually a pool of water there, with an island in the middle. The room also contained a bunch of slimy yellow-green writhing cocoons about 5' in diameter hanging from the ceiling, and smelled very strongly (strongly enough to force HT rolls) of rotting fish.

The combination of the ichor trail stopping abruptly, the illusion, and the sea monster not being here caused everyone to get very suspicious. Zaber doubled back and searched for secret doors, but didn't find any. A lot of discussion ensued, and while everyone was discussing, another leech swarm came out of the water and surprised the party. First they tried to get Seépravir, but she was Levitating a few feet up and they couldn't get through her (minimal) armor. Then they went for Polly, bleeding her a bit. At that point everyone recorded from surprise, Garreth started killing leeches with his katana, and Polly started killing leeches with her rapier. Soon the leech swarm was dispersed.

Not long after, a huge (SM+2) leech came out of the water at the group. This time they weren't surprised, so it wasn't much of a fight. Garreth smacked it a couple of times with his katana and stunned it, and then Polly put a couple of arrows into it, and then Garreth hit it some more and Polly shot it again and the giant leech stopped moving.

While everyone else was fighting leeches, Zaber was carefully observing the water, and he spotted a small humanoid, underwater, about 15 yards away. As Zaber started telling the others about it, the little guy fired a bolt of mana at Elias. Elias tried to dodge, failed, used his Luck, and succeeded.

At that point a ranged attack war began between Polly and the creature, that turned out to be a Derro Skum. Polly would fire arrows, accurately despite the penalty for firing into water, and then the Derro would dodge. And then the Derro would fire mana bolts at Polly, who would acrobatically dodge. Meanwhile another leech swarm came out of the water, but it failed to achieve surprise, and mostly just gave Elias and Garreth something to do while Polly and the Derro failed to hurt each other.

Meanwhile, Seépravir cast Swim and WarmthBreathe Water and Dark Vision on Garreth, enabling him to go swimming after the Derro without freezing to death or risking drowning or being really slow and heavily penalized. Garreth jumped in and started swimming at the Derro. Invisible Zaber also jumped in and started circling around behind the Derro.

Polly finally managed to put an arrow into the Derro Skum, though it only did half damage because of the water. Garreth swam within range and went for a Slam attack. The Derro Skum did a retreating Dodge, which kept in from getting Slammed, but put it within Zaber's reach. Zaber went for an All-Out Attack (Double), stabbing the Derro Skum twice in the skull. First one hit, second one was a Major Wound, and the Derro Skum, rolling Stun at -10 for a brain hit, failed by enough to be knocked out. That ended the fight. Garreth, in his usually Bloodlusty way, decided to decapitate the unconscious Skum and bring both parts back to shore for looting. It turned out to have nothing.

Zaber circled the pool, and didn't find any other secrets or exits. Seépravir cast enough Seek Magic to home in on whatever magic she found earlier, and found that there was something magic on the island. Zaber hadn't wanted to explore the island earlier because it was all slimy from slime dripping off the cocoons overhead, but magic was enough to motivate slogging through slime. Zaber found a bracer (below-the-elbow forearm armor), and a few minutes later found the other one. Seépravir found a magic ring. Zaber found several bags of salt, and then a (non-magical) shortsword. Seépravir cast Seek Magic again, and found no more magic in this room, with the nearest unknown magic a ways to the north.

Elias was pretty sure that the things in the cocoon were evil undead and all needed to die. Polly was happy to shoot one, and then they cut it open and found a dead derro halfway morphed into a skum. That was enough for everyone to agree with Elias, and kill all the others. This made the stench in the room even more overpowering, and Elias and Seépravir got badly nauseated and had to leave the room.

Seépravir checked out the suspiciously disappearing ichor trail, and decided it was probably an illusion. She cast Dispel Magic, and it went away!

At this point, the group discussed whether to press on down the underground river, explore the secret passage to the north, or go back to town. They decided on town.

When the group reached Cillamar, Garreth remembered to ask Quintus about the slimy green stuff they found in a glass box a few sessions ago. Quintus said it was demon toads eggs and would pay $200 for them. Garreth asked if they'd be used as potion ingredients, or hatched into demon toads. Quintus reassured Garreth that he had no intention of breeding demon toads, so the group agreed to sell the eggs. Zaber searched for Flitwick's servants again, but didn't find any.

GM's Comments:

The leech swarms were a challenge to one PC, but died pretty quickly when multiple PCs started damaging them. The big leech never managed to hit, as it was stunned then reduced to goo pretty quickly. The derro skum wizard abused the underwater combat rules nicely to keep plinking mana bolts at the PCs while mostly avoiding Polly's arrows, but once Garreth and Zaber both came into the water after it, it was doomed.

The party doesn't know if the sea monster was real or an illusion, but they can't find any sign of it, and the ichor trail it left was an illusion, so they're suspicious.

The illusion of a solid floor over a pool (containing leeches) was a nice simple trap, but Zaber's Perception was too good to be fooled. 3/5 of the PCs never saw the illusion, but they trusted Zaber and Seépravir, and didn't step where they were told not to step. I wish Roll20 let me draw two versions of the map, one for the players who saw through the illusion and one for those that didn't, but that's beyond its capabilities.

We quit 30 minutes early, because I warned the players that if they continued exploring, they might run into a fight that they couldn't finish this session and get stuck in the dungeon again. Which isn't a huge deal, but means they can't buy equipment or sell loot or spend character points.